Recently, the development of solar power generation by crystalline silicon solar cells has reached commercialization, following testing, due to the advantages thereof in view of non-pollution, safety, high performance, and reliability.
As a result, about several to several tens of MW of mass solar power generation has been made by using the silicon solar cells in Germany, Canada, USA, and the like.
The solar cell currently used in the solar power generation is produced by using a monocrystalline silicon ingot manufactured by the Czochralski pulling method or a polycrystalline silicon ingot manufactured by the Bridgman method. It is recognized that prices of the silicon ingot and substrate need to be lowered while product quality and productivity need to be further improved in view of future continuous increase in the capacity and improvement in economic feasibility.
By considering this background, considerable efforts have been made in efficiently producing high-quality polycrystalline or monocrystalline-like silicon ingot, of which physical properties do not significantly deteriorate as compared with those of monocrystalline silicon ingot and production costs are easily reduced.
Basically, the manufacture of the polycrystalline silicon ingot as well as the monocrystalline-like silicon ingot for the solar cell is characterized by directional solidification. Particularly, the monocrystalline-like silicon ingot is manufactured by further using monocrystalline seeds.
When the monocrystalline seeds are installed on the lowest portion of a crucible made of quartz or graphite and a solar cell-grade raw silicon fills the crucible, followed by heating at 1420° C. or more, all the monocrystalline seeds and the raw silicon are melted. Here, all the raw silicon is melted while a lower portion of the monocrystalline silicon seeds are partially not melted by controlling thereof, and then, when solidification heat of the silicon is removed in one direction toward a lower portion of the crucible, solidification spreads from the seeds at the lower portion of the crucible toward an upper portion of the crucible, following the process manner of directional solidification of monocrystalline-like silicon ingot.
In the case of polycrystalline silicon ingot, the ingot obtained by a well controlled directional solidification process has a columnar structure where a large number of monocrystalline columns are integrated in one direction. When the ingot is cut perpendicularly to a crystal growth direction, the resultant substrate has a structure where electrons generated by light are collectable toward an electrode without electron loss, in like the monocrystalline ingot.
As shown in Korean Patent Laid-Open Publication Nos. 2008-0068423, 2008-0068424, 2009-0035336, and 2009-0035337, recently, this polycrystalline silicon ingot manufacturing technology is further developed, and research and development of manufacturing an ingot having polycrystalline structure almost similar to a monocrystalline structure (a monocrystalline-like structure) have been made. That is, according to this technology, the monocrystalline seeds are used like in the growth of monocrystalline ingot, but the plate-shaped monocrystalline seeds are positioned on a lower portion in the existing polycrystalline silicon ingot crucible, and a monocrystalline ingot is grown. In order to selectively melt the monocrystalline seeds and successively grow the monocrystalline ingot, a special cooling system capable of selectively cooling only a part of the lower portion of the crucible or precisely controlling a heat transfer rate in time and space needs to be developed.
Currently, a commercial sized polycrystalline silicon ingot for a solar cell has about 400˜650 kg, and one ingot is manufactured for each batch in order to secure high quality. However, in order to manufacture a monocrystalline-like silicon ingot by using the existing polycrystalline ingot manufacturing apparatus, equipment such as a heat transfer system at the lower portion of the crucible or the like needs to be improved.
The technical point in the manufacture of a monocrystalline-like silicon ingot is that, when a raw material fills a lower portion in the crucible, a monocrystalline silicon seed is positioned in the lowermost portion in the crucible; at the time of melting the raw material, the melting is limited to only an upper portion of the monocrystalline silicon seed while a lower portion of the monocrystalline silicon seed is maintained in a solidified state; at the time of crystal growth, the liquid silicon is subjected to directional solidification toward a lower portion of the crucible. Here, there may be provided a good-quality ingot where the crystalline structure is a monocrystalline-like structure according to the seed used and the grain size is maintained similar to monocrystalline, resulting in significantly less crystalline defects and impurity mixing, that is, a monocrystalline-like ingot meeting the specifications of high-efficiency solar cells.
The performance of the ingot manufacturing apparatus of the related art mostly depends on the minimization in crystal defects and metal impurity mixing and maximization in columnar structure fraction, which are the main physical properties of the ingot, caused by reduction in process time required for heating and cooling.
The process factors, such as heating, cooling, and the like, are related to optimization of an apparatus such as a constitution of an insulating cooling system, and thus, arrangement of insulating materials and the like installed around a heater is important. In particular, the insulating cooling system around the crucible, which is contacted with a cooling means is very important. Therefore, it is necessary to construct a lower insulating and heat transfer system that is more elaborate than the related art, in order to manufacture a high-quality polycrystalline silicon ingot, further a monocrystalline-like, as well as other bulky semiconductor ingots or metal oxide ingots such as sapphire, that may be manufactured by a melt crystal growth method such as Czochralski.